In telecommunication systems in which adaptive echo cancellation is effected with the aid of an adaptive filter in a four-wire loop, in certain operational cases problems occur with self-oscillation, called "bursting". This phenomenon is caused by the presence in the four-wire loop of feedback from the output of the adaptive filter to its input, via the far-end of the loop. In this way, a near-end signal is able to pass around the loop and appear as a far-end signal at the filter input and thereby influence the setting of the adaptive filter. The risk of self-oscillation is greater in the case of shorter connections, for instance when so-called adaptive hybrids are used, than in the case of longer connections, for instance when so-called trunk echo cancelers are used. This is because the near-end signal passing around the loop is decorrelated as a result of the large delay that occurs in long connections.
The "bursting" phenomenon and a method of reducing the risk of "bursting", i.e. self-oscillation, is described in W. A. Sethares, C. R. Johnson J. R. and C. E. Rohrs: "Bursting in Adaptive Hybrids", IEEE Transactions on Communications, Vol. 37, No. 8, August 1989, pages 791-799.
According to the known method a so-called double-talk detector is used and the updating of an adaptive filter used for echo cancellation is interrupted when signals from a near-end are detected. The method can be relatively successful in the case of trunk echo cancelers which are localized in a network so that the echo path will be relatively heavily attenuated, e.g. more than 6 dB. However, attenuation of the echo is poorer in the case of an adaptive hybrid which lies in the actual 2/4-wire transition, and consequently the detector has a more difficult task. Furthermore, there is a risk that any self-oscillation will be interpreted as a strong near-end signal, causing updating to be interrupted. In this case, any self-oscillation that had commenced would become permanent, which is unacceptable.
It has also been found that when the signals are narrow band signals, e.g. when they are comprised of pure tones, a weak near-end signal may also have a pronounced disturbing effect on the adaptive filter, and therewith result in self-oscillation. Tone signalling is found, for instance, in modem traffic. Consequently, when applying the known method there is a risk that self-oscillation cannot be avoided when transmitting narrow band signals.
Another method for avoiding self-oscillation in conjunction with echo cancellation is described in J. W. Cook and R. Smith: "An integrated circuit for analogue line interfaces with adaptive transmission and extra facilities", British Telecom. Technol J., Vol 5, No. 1, January 1987, pages 32-42. According to this method, the adaptive filter is comprised of one of 24 predetermined filters. However, this merely provides highly limited possibilities of effectively canceling echoes. Consequently, it is probable that the method can only be used for adaptive hybrids, due to the relatively low requirement of echo cancellation that then exists.
It is also known, in conjunction with adaptive control, to enhance the stability of an adaptive system by limiting the permitted range for the coefficients of an adaptive filter. In this regard, it is necessary to be aware of the requirements that the adaptive filter must fulfil in order for the system to be stable, and to ensure that the coefficients are restricted in a manner which will fulfil these requirements. When the requirements for preventing self-oscillation are known and can be transferred to the coefficient space, i.e. to the value of the coefficients, it is possible, of course, to guarantee stability. However, a difficulty resides in finding a criterion which can readily be transformed to the coefficient space and which does not require undue calculation and which will not excessively restrict the permitted range. Excessive limitation of the range would unnecessarily impair performance under the normal conditions in which no such limitation is required.
One example of the restriction of the permitted range for filter coefficients is found described in A. Krieger and E. Masry: "Constrained Adaptive Filtering Algorithms: Asymptotic Convergence Properties for Dependent Data", IEEE Transactions on Information Theory, Vol. 35, No. 6, November 1989, pages 1166-1176.